This invention relates to curing agents for epoxy resins. In a specific aspect, the invention relates to phenolic curing agents for epoxy coating compositions.
Bisphenol-A (BPA) terminated epoxy resins are useful as solid curing agents for epoxy powder coatings. Such phenolic curing agents are conventionally prepared by reacting about a 65:35 weight ratio of BPA with a low molecular weight liquid epoxy resin to produce a reaction product having a weight per phenolic of about 270 and made up primarily of the relatively low molecular weight BPA-terminated liquid epoxy and unreacted BPA. The uncatalyzed reaction of these phenolic curing agents with epoxy resins is very slow, and a catalyst such as 2-methylimidazole (2-MI) is often incorporated by melt-blending into the molten curing agent at the end of the production sequence.
Because of an industry need for faster curing powders, higher levels of 2-MI are desirable. It has been found, apparently because of a salt formation between 2-MI and phenolic hydroxyl, that these higher levels of 2-MI yield low melting point mixtures which tend to sinter during storage. Resins or curing agents which sinter can cause problems during application as powder coatings. Examples of such problems are the blockage of application equipment due to impact fusion and the formation of "cobwebs" during electrostatic application to hot objects.
It is therefore an object of the invention to prepare phenolic curing agents which have a weight per phenolic of approximately 270 and, when combined with imidazole cure accelerators, exhibit reduced tendency to sinter during storage.